3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) and the evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. Such an E-UTRAN typically comprises wireless communication devices (wirelessly) connected to network nodes, commonly referred to as eNodeB. An eNodeB serves one or more areas referred to as cells.
Positioning is the process of determining coordinates in space. Once the coordinates are available, the position can be mapped to a certain place or location. The mapping function and the delivery of the location information on request are parts of the location service which is required for the basic emergency services. Services that further exploit the location knowledge or that are based on location knowledge to offer customers some additional value, are referred to as location-aware and location-based services, respectively.
There exist a variety of positioning techniques in wireless communications networks, differing in their accuracy, implementation cost, complexity, applicability in different environments, etc. In existing networks, the most common are solutions assisted by a wireless communication device where a serving mobile location center (SMLC in GSM and UMTS, enhanced SMLC (eSMLC) in LTE) calculates the wireless communication device position based on measurements reported by the wireless communication device.
The SMLC/eSMLC is either a separate network element (as illustrated in FIG. 2) or an integrated functionality e.g. in a network node such as a RBS (Radio Base Station). Among such methods, Assisted Global Positioning System (A-GPS) typically provides the best accuracy. Combining the mobile technology and GPS, A-GPS enhances the receiver sensitivity by providing orbit and other data to the wireless communication device. Drawbacks of A-GPS is that a GPS-equipped wireless communication device is required, and that it doesn't function in certain environments such as tunnels, indoor areas and dense urban areas. Therefore other complementing methods for positioning are needed. These methods use measurements of the time difference of arrival (TDOA) of signals between the cellular antenna and the wireless communication device. In UMTS observed TDOA (OTDOA) is used. In GSM a variant called Enhanced Observed Time Difference (E-OTD) is used.
The technique currently adopted for LTE-based positioning is OTDOA. OTDOA is a multi-lateration based technique estimating TDOA of signals received from three or more sites. To enable positioning, the wireless communication device should be able to detect signals from at least three geographically dispersed network nodes. This implies that the signals need to have high enough signal-to-interference ratios (SINR). Furthermore, the signals need to be transmitted frequently enough to meet the service delay requirements. In order to meet the accuracy requirements, the signals may need to be accumulated over multiple sub frames.
To enable positioning measurements in LTE, a straightforward solution would be to measure standardized signals that are always transmitted from a network node, e.g. synchronization signals (SS) or cell-specific reference signals (RS). SS and cell-specific RS (CRS) are physical signals used to support physical-layer functionality and they do not carry any information from the Medium Access Control (MAC) layer. Both signals are transmitted according to a pre-defined pattern, i.e. in selected subcarriers and time slots, and the pattern is typically relatively sparse.
In LTE, SS are transmitted in downlink and are primarily used in the cell search procedure, i.e. for the wireless communication device to identify a cell and synchronize to it in downlink in order to read the broadcast channel information. SS are transmitted in sub frame 0 and sub frame 5 of a radio frame. A SS consists of Primary SS (PSS) and Secondary SS (SSS). First, a cell identity is read from PSS, and then the cell identity group is read from SSS. The cell identity can then be used to determine the CRS sequence and its allocation in the time-frequency grid. The SS occupy 62 resource elements in the center of the allocated bandwidth.
CRS are transmitted over the entire system bandwidth and in every sub frame, i.e. more frequently than SS. In normal sub frames with a normal cyclic prefix where each time slot comprises seven OFDM symbols, CRS are transmitted on resource elements (RE) of a time-frequency resource grid for one sub frame in time and 12 subcarriers in frequency (the number of subcarriers corresponding to a physical resource block (PRB)). In a system with a single transmit antenna up to six different shifts in frequency (frequency reuse factor=6) and 504 different signals can be used for the CRS. With two transmit antennas, the maximum frequency reuse factor reduces to three. With four transmit antennas, the possibilities are even more limited. Other CRS patterns are defined for sub frames with extended cyclic prefix and for multicast broadcast single frequency network (MBSFN) sub frames.
However, it has been shown that using SS and CRS for positioning without interference management would result in positioning coverage problems due to low SINR and/or insufficient number of strong signals from different network nodes. The problem is particularly relevant for synchronized networks or networks with high data load, as there is a high probability of parallel transmissions in multiple cells on the RE used for CRS or SS which leads to high interference. Furthermore, the SS transmission frequency is not sufficient for the positioning requirements.
To improve positioning measurements and address the hearability problem, it has been proposed in 3GPP to introduce positioning RS (PRS), which could be designed according to transmission patterns characterized by a lower collision probability. In general, PRS may or may not be transmitted in multiple consecutive sub frames and the periodicity can be configured statically or semi-statically.
In respect to the frequency dimension, given a PRS transmission pattern per PRB, the simplest solution would be to repeat the same pattern in all PRB of the same sub frame, i.e., over the entire bandwidth. Transmitting PRS over a large bandwidth generally improves positioning accuracy due to a higher measurement resolution and a lower probability of being in unfavourable frequency-selective fading conditions. The drawback is that a large bandwidth gives a high wireless communication device complexity. Furthermore, a smaller bandwidth may be sufficient to achieve the required accuracy, and using the entire bandwidth is then a waste of resources.
At a high system load, there is no gain in introducing the new PRS without interference coordination. One of the approaches for reducing interference is to transmit PRS during low-interference sub frames (LIS) in which PDSCH transmissions are suppressed. In the LIS, there are RE used for PRS, RE used for control signalling, but the rest of the REs are free from data transmission. To achieve an even higher interference reduction, LIS can be aligned among the cells. For the LIS alignment, inter-cell coordination may or may not be needed, depending e.g. on if LIS occurrences are configured statically or dynamically. However, PRS collisions may occur if cells employ the same transmission pattern.